Duplexers, which are used, for example, in portable communication appliances, on the one hand have the purpose of passing on transmission signals from transmission paths to an antenna connection without them being passed into reception paths, which are intended for received signals and have sensitive low-noise amplifiers. This means that duplexers thus must have the capability to carry a large amount of power in the transmission path at the transmission frequencies and, ideally, a low insertion loss in the pass band, since this is the only way to achieve a communication appliance with a low power consumption. High suppression of the transmission path at the reception frequencies is required in order to protect the reception path against the high levels of the transmission signal.
Duplexers should likewise pass received signals from an antenna connection into a reception path with as little further attenuation as possible in order to ensure high reception path sensitivity even at low useful signal levels. For this purpose, a low insertion loss is also a necessary precondition for the reception filter.
Furthermore, an antenna duplexer is intended to prevent the emission of transmission power in undesirable frequency ranges either leading to interference in other radio systems or to interference being produced in the device itself at frequencies at which signals at very low levels are intended to be received, for example, GPS. In order to achieve this, high suppression is required in the transmission path in the relevant frequency ranges (for example, GPS, WLAN, harmonics of the transmission signal).
Correspondingly, therefore, duplexers should ideally have a low insertion loss in respective pass bands, with high suppression in the other frequency ranges. In the mobile radio field, SAW or BAW duplexers (SAW=surface acoustic wave, BAW=bulk acoustic wave) are preferably used because of the filter characteristic which is achieved in a very small physical form.
For example, one duplexer circuit is known from the document U.S. Pat. No. 7,053,731 B2.
For antenna duplexers in the mobile radio bands in the 2 GHz range (for example, Bands I, II, III, IV or VII), simultaneous compliance with all the requirements stated above is more difficult than in the 1 GHz range.
For SAW components, it is not possible to use solely a DMS filter structure (double mode SAW), which leads to low insertion losses and high broadband suppression levels even while away from the pass band frequencies, for the transmission filter, because DMS structures do not have the required power handling capability, in particular in the 2 GHz range. So-called “ladder-type” structures are therefore used for the transmission filter.
For BAW, the use of so-called coupled resonator structures involves an extremely complex process, and the ladder-type structure is therefore currently also used here.
However, the use of the ladder-type structure for the transmission filter fundamentally has the disadvantage that the transmission filter therefore has inadequate broadband suppression at the required level for the insertion loss.
In addition, the dissipated losses which increase at high frequencies make it harder to achieve low insertion losses. The measures to reduce the insertion loss that are known for ladder-type structures are on the one hand to reduce the capacitance ratio of the parallel to series resonators and on the other hand to reduce the number of ladder-type basic elements used. However, both measures lead to a reduction in the broadband suppression level, which is therefore contrary to the requirement for the transmission filter. If the above measures were to be implemented in the opposite sense in order to achieve the required suppression, the insertion loss would be increased, and the bandwidth and gradient in the bandpass filter would be reduced.
For narrow band selection requirements above the transmission band, additional suppression can be achieved by measures such as shifting pole points above the transmission band—as described in the German patent document DE 199 32 649 A1.
However, the options for narrow band selection requirements well below the transmission band are very restricted. The GPS frequency range (1574-1577 MHz) is, however, actually below the device's own transmission range for a 2 GHz duplexer.
One previously known measure for increasing the suppression outside the pass band frequencies of duplex circuits, in particular of duplexer circuits which are intended for use at mid-frequencies in the region of 2 GHz, which are intended to have high suppression of the transmission signals at GPS frequencies, is, for example, based on deliberately positioning acoustic poles below the pass band frequencies for GPS frequencies. One disadvantage is that an additional reactance element is required for this purpose, typically an inductance. This inductance has to be chosen to be greater the further the acoustic pole point is intended to be away from the pass band. A further disadvantage is that the suppression decreases considerably in the frequency range between the pass band and the frequency of the acoustic pole.